This invention relates to stimulation of subterranean wells. More particularly it relates to matrix stimulation by acidizing and to acid fracturing. Most particularly it relates to a new method of diverting injected acids to improve zonal coverage.
Acidizing is a method in which an acidic fluid or a reactive fluid is contacted with a subterranean formation (the “matrix”) penetrated by a wellbore. The acidizing fluid contacts and dissolves wellbore damage and/or part of the matrix. If the treatment is applied above the fracturing pressure, the injected fluid fractures the rock and the principle function of the acidic fluid is to create wormholes and/or to differentially etch the opposing faces of the fracture so that when the fracture closes when the injection pressure is released, the faces no longer match up and flow paths remain running along the fracture faces from the fracture tip to the wellbore to conduct formation fluid into the wellbore for production. It is important that the injected fluid should reach all of the target zone for maximum beneficial effect. This is difficult to achieve because of a natural tendency of the acid to react with the first reactive formation rock with which it comes into contact (because it is nearest to the wellbore or because it is the most porous or because it is the most accessible due to natural fractures or vugs) in either matrix acidizing or acid fracturing. Depending upon the heterogeneity of the rock, the reaction rate of the acid with the rock, and the rate at which fresh acid is delivered to the rock, the acid reaction may be relatively uniform, may form one or a few long wormholes extending into the rock, or may form a network of many smaller wormholes extending into the rock. All of this is well known to those of experience in the art. Attempts to achieve complete contact of acid with an entire rock formation zone (termed zonal coverage) involve diversion of acid from the regions first contacted to new regions. This is because otherwise the acid will tend to continue to react with the first rock with which it comes into contact, especially because it will have formed preferential flow pathways for subsequently injected acid. Diversion is also necessary when the formation is made up of strata having different permeabilities. When there is a permeability contrast, the initially injected acid will tend to enter the most permeable layer or layers first and in fact increase their permeability further, and then will continue to enter those layers. Diversion will correct this problem.
Zonal coverage may be achieved either by applying a mechanical method such as injection through coiled tubing with portions of the target formation successively isolated with packers, or by placing a fluid (such as a gel or a foam) or an additive (such as a salt) after treatment of a zone or part of a zone, which impedes fluid flow into the treated zone and diverts the acid or reactive fluid from the treated zone to a new (not yet treated) zone.
Foamed fluids have been shown to be able to block a formation not just by their viscosity but also by the mechanism of breaking and reforming under dynamic flow conditions. Furthermore, foamed fluids will block a formation more effectively the greater the bubble size in the foam relative to the pore size. When there is stratification (layers of varying permeability), diversion is achieved by generating and maintaining a stable foam in the higher permeability zone or zones during the entire treatment. When there is a long zone to be treated, diversion is achieved by treating part of the zone with acid, then placing a foam to block entry of subsequently injected acid into that part of the zone, and then injecting more acid. These alternating steps may be repeated. The result is complete zonal coverage by the treating fluid and effective damage removal by the acid, even from severely damaged zones. Depending upon the type and concentration of the surfactants used and the foam quality, foams can generate different levels of yield stress. Foamed fluids have also been known to support solid particles and to enhance the stability and viscous flow behavior of fluids. Foamed fluids have also been recognized as one of the best diversion fluids for acid stimulation. Other benefits of foamed fluids are that they are inherently cleaner than non-foamed fluids, even if they contain polymers, because they contain less liquid and that they help kick off flow back and clean up because they provide energy to the system to help overcome resistance, for example the hydrostatic head, to flow back. That they are “energized” is particularly important in depleted reservoirs.
Gelled fluids are used as diverters when they are injected already gelled with a polymer (that may, in addition, be crosslinked) or with a viscoelastic surfactant system. These fluids divert in the same way as do mechanical devices, or chemicals such as salts, by being placed where it is desired to impede the flow.
A new technology, viscoelastic surfactant gel systems, has also been shown to be useful in diverting an acid or a reactive fluid in a new way. In this case, when formulated properly (depending upon the nature and properties of additives and of the surfactant system used), the viscoelastic surfactant fluid is initially acidic and of low viscosity and this fluid then “gels” (increases in viscosity) after the acid in it has spent and the pH increases and thus it temporarily reduces the injectivity of subsequently injected fluids into a zone after stimulating it. These materials (the surfactants in acid) are sometimes known as “viscoelastic diverting acid systems” or “VDA systems” and can be used for fracture stimulation and for acidizing. We will term fluids that have been viscosified with viscoelastic surfactant systems as “gels” or “gelled”. These fluid systems exhibit self-diverting behavior as they gel when the acid spends. Typically the viscosity change during acid spending is in the range of 5 to 300 cP (at 170 s−1)_depending on the temperature. Thus, as injected, they have low viscosity and they enter and react with the first reactive matrix material with which they come into contact, but after they react they gel and plug up that region of the formation, forcing subsequently injected fluid to enter a new region of the rock matrix. This is sufficient to give the material self-diverting characteristics. This enables subsequently-injected acid or reactive fluids to further stimulate the other oil or gas zones, or to increase the sweep in water or gas injector wells. After the treatment the diverter gel is destroyed either by flowback fluids or by an internal breaker.
These techniques can be applied in any situation in which it is difficult to contact all of the target matrix. For example, in vertical or deviated wells the target formation could be stratified into layers that have different permeabilities (or different reactivities to the acid or reactive fluid) or the target formation could be so thick (from top to bottom) that for one or more of several reasons it is difficult to contact all of the target in a single treatment. Diversion techniques could also be applicable and necessary in horizontal wells; in such cases, the formation might not be thick but the distance along which a wellbore penetrates the formation may be great, so it would be very difficult for injection of acid made in a single stage to reach the far end of the wellbore penetrating the formation.
Although there are many methods known for acid diversion, they may require expensive and complicated equipment and time-consuming operations if they are mechanical. If they are chemical they may be inefficient and give incomplete zonal coverage, often require many additives, many steps, and large amounts of materials, and then may require time and additional chemical treatments for their removal. Thus there is a need for a simple, inexpensive, fast, reversible method of effective diversion.